1. Field of the Invention
The present invention relates generally to aerosol sampling methods and devices and, more particularly, to high volume devices for sampling concentrations of particulate matter in ambient air. Specifically, the present invention relates to a method and apparatus for controlling flow volume and maintaining a constant mass and volumetric flow through such aerosol sampling devices.
2. Description of the Prior Art
It is recognized and generally accepted that gaseous air pollutants are deleterious to the health of persons. Scientists are also aware that particulate pollution in air has serious adverse health effects. The U.S. Environmental Protection Agency (EPA) has set standards for particulate matter in air in terms of mass per unit volume limits over a preselected period of time. For example, present standards for particulate matter are 75 micrograms per cubic meter average annual limit (geometric mean) and 260 micrograms per cubic meter in 24 hours (geometric mean) for particles up to a normal size range of 25 to 45 microns.
New data has become available which indicates that protection of public health may be better served by considering only those particles which are inhalable. The health risks posed by inhaled particles are influenced both by penetration and deposition of particles in various regions of the respiratory tract and by biological responses to these deposited materials. More specifically, it has been found that the risks of adverse health effects associated with deposition of ambient fine and coarse particles in the thorax (tracheobronchial and alveolar regions of the respirtatory tract) are markedly greater than for deposition in the extrathoracic (head) region. Maximum particle penetration to the thoracic region occurs during oronasal or mouth breathing. Further, it has been found that the risks of adverse health effects from extrathoracic deposition of general ambient particulate matter are sufficiently low that particles depositing only in that region can safely be excluded from the standard indicator. Consequently, the size-specific indicator for primary standards should represent those particles capable of penetrating to the thoracic region, including both the tracheobronchial and alveolar regions. As a result, the International Standards Organization has proposed a standard based upon particles deposited on the tracheobronchial regions of the human respiratory tract. This proposal is now referred to as the thoracic deposition TPC (Thoracic Particles).
The Clear Air Scientific Advisory Committee (CASAC) has now recommended to the United States Environmental Protection Agency (EPA) that a 10 micron particle size range be used as the new primary standard for average annual limits and 24 hour limits of micrograms per cubic meter clean air standards. Therefore, in accordance with sections 108 and 109 of the Clean Air Act, the EPA has reviewed and revised the criteria upon which primary and secondary particulate matter standards are based. The existing primary standards for particulate matter (measured as "total suspended particulate matter" or "TSP") have been 260 .mu.g/m.sup.3, averaged over a period of 24 hours and not to be exceeded more than once per year, and 75 .mu.g/m.sup.3 annual geometric mean. The secondary standard (also measured as TSP) has been 150 .mu.g/m.sup.3, averaged over a period of 24 hours, and not to be exceeded more than once per year.
As a result of its review and revision of the health and welfare criteria, the EPA has now proposed several revisions to its particulate matter standards. First, the EPA proposes that TSP as an indicator for particulate matter be replaced for both of the primary standards by a new indicator that includes only those particles with an aerodynamic diameter smaller than or equal to a nominal 10 micrometers (PM.sub.10). Second, the EPA proposes that the level of the 24 hour primary standard be changed to a value to be selected from a range of 150 to 250 .mu.g/m.sup.3 and that the current deterministic form of the standard be replaced with a statistical form that permits one expected excess over the standard level per year. Third, the EPA proposes that the level and form of the annual primary standard be changed to a value to be selected from a range of 50 to 65 .mu.g/m.sup.3, expressed as an expected annual arithmetic mean. Fourth, the EPA now proposes that the current 24 hour secondary TSP standard be replaced by an annual TSP standard selected from a range of 70 to 90 .mu.g/m.sup.3, expected annual arithmetic mean.
As a consequence of these changes, a need exists to develop monitoring instruments that mimic the deposition of particles in the thoracic region of the human respiratory system. High volume sampling techniques to determine the amount of particulate matter within gases such as air are well known. 40 CPR Part 50, Appendix B, as amended by the EPA in the Federal Register, Volume 47, No. 234, Dec. 6, 1982, discloses reference methods and monitors for determining total suspended particulates in the atmosphere. The high volume sampler method disclosed therein is adapted to collect large samples to enable sufficient matter to be collected for analysis on a 4-place balance, and this technique has found wide acceptance in the industry.
However, the reproducibility and accuracy of particle concentration determined with such a high volume sampler has often been overlooked. In prior art high volume sampler devices, the blower motors used to draw the aerosol into the samplers generally have characteristic flow rate performance curves which show progressive decreases in the flow rates through the filters of the samplers as a result of increasing particulate accumulation on the filters. Thus, the flow rate in such a prior art device is not constant or known during the sampling period. Other variations in the flow rate of sampling can occur due to line voltage variations in the electrical circuits leading to the motor, temperature and pressure changes of the ambient air which significantly alter the pumping rate of the high volume sampler motor, and motor/blower performance degradation.
The EPA has now recognized that significant errors in the air volume determination can result from errors in flow rate and/or sampling time measurements. Therefore, the proposed rules promulgated by the EPA on Mar. 20, 1984, in 40 CFR Part 50 would require ambient aerosol samplers used to monitor compliance with the EPA clean air standards to be equipped with an automatic flow control device capable of maintaining the sample flow rate within certain specified limits.
Attempts have been made to regulate such high volume sampler devices to maintain constant air flow, which would require flow controllers that are capable of maintaining a flow rate independent of filter loading, temperature and pressure changes, as well as line voltage variations. Such a flow controller, of course, would improve the accuracy, representativeness and reproducibility of the measurements. However, the prior attempts to provide such flow controllers have not been very effective or successful. The devices which have been utilized in the past have suffered from a number of deficiencies, which include, among other things, the use of precision mechanical systems based on maintenance of a constant back pressure across an aperture, pressure tap or a capillary tube. All of these devices have been temperature sensitive and therefore require temperature-regulated enclosures or complicated temperature compensating devices for proper operation, none of which are completely satisfactory for long term, reliable, maintenance free use.
Some of the prior devices have also used the discharge pressure of the motor/blower as a measure of flow rate. While this kind of approach is apparently an attempt to a straightforward solution, such a system introduces another error. As the speed of the blower motor is increased to compensate for filter loading by particulate matter accumulating thereon, an increasingly heavy load is placed on the motor. As the motor thus becomes more and more heavily loaded, heat is generated which increases the temperature, and therefore the pressure, of the exhaust air. Thus, the discharge pressure of the blower motor is increased, which causes the control system to change the rate at which the motor operates. This resulting change is not in response to a change in the condition of the ambient air, but is instead a change caused by the heat of the blower motor. Consequently, this resulting changed speed of the blower motor causes an error in the control of the air flow through the sampler device.
Other attempts have been made to regulate the mass flow rate through such aerosol sampler devices. One specific device is the Kurz flow controller disclosed in U.S. Pat. No. 4,067,705. Laboratory studies have confirmed that this device is a constant mass controller and, when operating correctly, it will control the flow rate at a constant mass. However, such correct operation only occurs at a set point reference condition such as the 25.degree. C. set point conditions specified by the EPA for calibration. It does not occur at normal ambient operating conditions encountered in the field. This limitation poses a distinct problem due to the fact that the above described amendments to 40 CFR Part 50 proceed on the assumption that flow controllers will yield constant flow at actual ambient operating conditions, not merely at reference conditions. It has been demonstrated that errors in actual volumetric flow rate of 25% to 30% can result if the flow controller is operated at actual ambient temperatures differing from the reference setpoint temperature, due to both diurnal and seasonal temperature variations. Thus, the value for total volume of air sampled used in calculating ambient particulate concentration levels will bear little resemblance to the actual volumes sampled. Also, the performance of particle size-specific inlets will not experience their designed linear flow rate, and sampler cutpoints for particle size could be adversely affected so as to not meet the EPA proposed regulations discussed above.